1. Field of the Invention
The present invention relates to a catalyst and a method of preparing the same. More particularly, the present invention describes a supported catalyst and a method of preparing the same.
2. Description of the Related Art
Supported catalysts are widely used to accelerate the chemical reaction rate in various applications. It is well known that a supported catalyst consists of a catalyst component and a porous support material component. The catalyst component is attached to a surface of the porous support material component. In general, since many pores are present in a porous support is material, the porous support material has a very large surface area. Such a large surface area provides many positions in which many catalyst particles can be dispersed.
For example, a carbon-supported metal catalyst uses a porous carbon particle as a support material and a catalytic metal particle as a catalyst. Such a carbon-supported metal catalyst is used in an electrode of a fuel cell. More specifically, a carbon-supported metal catalyst is used in a cathode and/or an anode in a phosphoric acid fuel cell (PAFC), a proton exchange membrane fuel cell (PEMFC), or a direct methanol fuel cell (DMFC).
The catalyst accelerates the rate of electrochemical oxidation of a fuel and/or the rate of electrochemical reduction of oxygen. The carbon particle of the carbon-supported metal catalyst has a dual role as a support material and as an electronic conductor. Platinum and platinum/ruthenium alloy, for example, are frequently used as a catalytic metal particle.
Typically, a supported catalyst is prepared by adding a catalytic metal precursor solution to a dispersion of support material so that the catalytic metal precursor adsorbs onto the support material. Then a solution of a reducing agent is added to the dispersion to reduce the catalytic metal precursor adsorbed on a surface of the support material to a catalytic metal particle (see, U.S. Pat. No. 5,068,161). Finally, a freeze-drying is performed to obtain supported catalyst powders.
It is well known that one of main factors that affect the catalytic activity of a supported catalyst is the total surface area of supported catalytic metal particles. The surface area of the catalytic metal particles is, in turn, affected by the average particle size of catalytic metal particles and the amount of the catalytic metal particles present on the support. If the amount of the catalytic metal particles on the support is constant, the average particle size of the catalytic metal particles is inversely proportional to the total surface area of the supported catalytic metal particles. If an average particle size of the catalytic metal particles is constant, the amount of the catalytic metal particles on the support is directly proportional to the total surface area of the supported catalytic metal particles.
Thus, one of the important technical objects in the supported catalyst field is to produce smaller supported catalytic metal particles than a conventional supported catalyst.
In a fuel cell such as a PAFC, PEMFC, or DMFC, as the activity of a carbon-supported metal catalyst contained in the electrode increases, the power density of electricity generation in the fuel cell increases while maintaining energy efficiency. Accordingly, the ratio of power generation to production costs of the fuel cell stack increases and the ratio of power generation to the weight or volume of the fuel cell stack increases.
For a supported catalyst prepared using the conventional method, as the amount of the catalytic metal particles on a support increases, the average particle size of the catalytic metal particles tends to increase as well. Due to this phenomenon, there is a limit on the improvement of the catalytic activity of a supported catalyst.
Further, although the size of the catalytic metal particles on the support can be reduced by preparing them according to the conventional method, the decrease of the average particle size of the catalytic metal particles also has a limit.
Thus, there is a need to develop a technique that can reduce the average particle size of catalytic metal particles to be placed on a support material while increasing or maintaining the conventional amount of catalytic metal particles supported.